18650 Batteries In Parallel

Introduction

The 18650 is a specific type of rechargeable lithium-ion cell battery referred to by its proper name the “18650 cell”.  The name 18650 refers exclusively to the size of the lithium-ion battery cell which is 18mm x 65mm and is slightly larger than an AA battery. The 18650 has become the de facto standard for replaceable and rechargeable batteries.  A typical lithium-ion cell has performance capacity in the range of 1800mAh to 3500mAh, an output of 3.7 volts as well as an average battery charge time of about four (4) hours. The charge time tends to differ with voltage of the charger, the battery type as well as the amperage.

The 18650 cell falls into 2 broad classifications namely the protected and unprotected. The unprotected variety is often used by very experienced users who build battery packs or use the 18650 for electronic cigarette and vaping purposes. These require higher discharge rates than protection circuits allow. Protected 18650 batteries are recommended for use because they have an electronic circuit embedded in the cell packaging (battery casing) which protects the cell from heat, over-charge, over discharge, over current and short circuit.

This makes it safer than the unprotected option. Unprotected 18650 batteries are cheaper (but less safe) and should only be used when load/draw and charging is externally monitored and controlled. In terms of power, an 18650 3.7v 3400mAh battery stores about 2 Ah to maximum 3.5 Ah and about 10 to 13 watt-hours. The best low cost 18650 battery is the Panasonic NCR 18650b which is an 18650 3.7v 3400mAh protected cell. One of the best chargers on the market is the Nitecore D4 which charges a wide variety of batteries.

The 18650 offers the best performance of any consumer grade rechargeable battery of its class. It tends to degrade at about the same rate as a smartphone battery, but is not prone to damage from charging, before being fully discharged (unlike the old nickel cadmium cells previously used widely). The most important parameter to verify when making an 18650 battery selection is its Continuous Discharge Rating (CDR) also known as the amperage capacity.  The CDR is the rate at which current measured in amps (A) can be pulled from the battery without overheating it.

It is important to match the CDR of a battery with the power draw of your device. If an error is made in this regard, the cells will tend to overheat which will damage the battery and reduce its lifespan. This in turn can cause the cell to leak, explode or damage devices. An inverse relation exists between the CDR (A) and battery capacity (mAh) namely: the higher the capacity the lower the CDR. Consequently, devices that draw less power can utilize higher capacity cells while devices that tend to consume more power will require lower capacity cells in order to safely draw more current. As at June 2018, the current maximum CDR attainable in an 18650 battery is 38A at 2000 mAh.

The best batteries are generally produced by Samsung, LG, Sony and Panasonic/Sanyo which brands provide reliable CDR ratings.  A good quality 18650 battery charger recommended is the Nitecore I2 Intellicharge charger which charges two cells at once. It detects the battery status then changes the voltage and appropriate charge as the occasion may demand. This helps prevent damage due to overcharging.

On the average, the 18650 battery tends to last from 300 to 500 charge cycles but the battery lifetime can be extended if it is charged regularly before complete discharge. Consequently, it is usually possible to get a year or two of continuous functional use for the 18650 battery, before it degrades as all cells inevitably will over time. It is advisable to select an 18650 battery that is slightly above the minimum discharge requirement for the device it is to be used for, while also focusing on the charge capacity.  Most batteries have in-built protection circuit boards (PCBs) which monitor the battery’s discharge rate and internal temperature.

It is beyond doubt that lithium-ion batteries of the 18650 cell variety have revolutionized portable devices. The 18650 cells are utilized in flashlights, laptops, smartphones, cameras, baby monitors, fitness gadgets, laser pointers and high drain devices. This is due to their superior capacity and discharge rates. 18650s come in both flat and button top styles.

In order to fully expand on the 18650, its use, advantages and operating capabilities in parallel, the following key points or questions will be considered namely:

· Can you parallel two different batteries?

· What happens when you connect 18650 batteries in parallel?

· Can you charge 18650 batteries in parallel?

Can you parallel two different batteries?

In order to properly address this, the concept of a battery bank must be understood. A battery bank is simply what you get, when you join two or more batteries together for a single application. When batteries are connected together this way you can either increase the voltage, amperage or both. There are two primary ways to successfully connect two or more batteries. The first is via a series and the second is called a parallel connection.

A parallel connection will increase your current rating, but the voltage will stay the same. It also has to be taken into consideration, that because the amperage (current rating) of the batteries have increased, you will likely need a heavier-duty cable to keep the cables from burning out. In this (parallel) arrangement a jumper wire is used to connect both positive terminals, and another jumper wire to connect both the negative terminals of both batteries to each other. Negative to negative and positive to positive. However, it is a different issue when you want to connect in parallel two different batteries. This is because different batteries means the batteries have different voltage ratings and capacities. Therefore, different batteries should not be connected in parallel. If this is done, the battery with the higher voltage rating will discharge into the other one (with the lower voltage rating) until they end up with equal voltages. If the second battery (with the lower voltage rating) is a rechargeable battery, then it will be charged by the first one, again until the two have the same voltage. What happens is that the end voltage will be intermediate between the two differing starting voltages. This will result in the current flowing between both batteries to be of a very high value. Inevitably, this high current will damage one or even both of the batteries. In the process a lot of heat will be generated very rapidly, which likely will lead to violent results as the batteries could or will end up being boiled (consumed).

What happens when you connect 18650 batteries in parallel?

It is important to clarify from the onset that it is not recommended that batteries of different type, age or capacity are mixed together and there are valid reasons for this. Newer batteries for instance, charge at a faster rate than older batteries, which could lead to the newer being overcharged while the system tries to fully charge the older. This would damage the newer battery causing it to fail and eventually bring down the whole battery bank. This is because different age batteries will charge and discharge at different rates, which will lead to an overall shortened battery bank life, when compared to a configuration with batteries of the same age. This could result in the batteries being damaged irreversibly.

It is recommended that batteries to be connected are all of the same age, type and capacity. The 18650 battery or cell comes in various flavours (voltage ratings and capacities). Therefore only 18650 batteries of the same age, voltage rating and capacity should be connected together in parallel. If 18650 batteries of the same type, age, size, capacity and voltage rating are connected in parallel it will result in increased capacity which should give a longer runtime, however the voltage remains the same.

Can you charge 18650 batteries in parallel?

It is possible to charge 18650 batteries in parallel safely, provided the following conditions are met: all batteries must be nominally identical (which means they must be of the same age, type, voltage rating, size and capacity). If this condition is met then for a parallel arrangement the charging current will divide equally. If this is not the case (if the cells are not identical), then the current will not divide equally, and in the worst of scenarios full current from the charger will all go into one cell at some point during the charging process. If this occurs, and the maximum charging current does not exceed the rated maximum current value for one cell, no damage will result. However, if the charging current is higher than that rated for the cell then damage will occur as this will destroy the lithium ion cells.  This will in turn compromise the battery bank as a whole and ultimately result in failure. In order to ensure 18650 batteries are charged safely in parallel, safety precautions should be taken. The maximum current into a cell during charging should be limited. A safe charging current as little as one-fifth (1/5) of the nominal capacity and not exceeding one-half (1/2) should be applied via an independent over-charge prevention circuit. This is a first line of defense. Secondly, it is recommended that protected 18650 lithium ion batteries be used for additional protection. Thirdly, it should be ensured that the charger circuit itself comes with inbuilt over protection so as to safely regulate the maximum voltage applied to the cells. For maintenance and storage it is recommended that a charger with at least 2% -3% of the capacity of the battery bank is used. Charge and discharge must be even. Fourthly, the cells must also be protected from over discharge by a protection IC which also protects from over charge. If the battery cells are not adequately protected their operation would be greatly impaired and their lifespan shortened.